PROJECT SUMMARY Although several lines of evidence support the use of immunotherapy in triple-negative breast cancer (TNBC), the modest clinical efficacy achieved in current clinical trials suggests that the immunosuppressive microenvironment cannot be overcome by PD-1/PD-L1 blockade alone. In order to improve outcomes, this project will investigate the immunologic effects of two emerging classes of targeted breast cancer therapies, poly (ADP-ribose) polymerase (PARP) and BET bromodomain (BBD) inhibitors, and will test the hypothesis that combinations of these agents with immunotherapies will be effective therapeutic strategies for BRCA-mutated and sporadic TNBC. The rationale for this work is based on our preliminary data indicating that PARP inhibition can activate the STING pathway, alter tryptophan metabolism and stimulate the infiltration and activation of cytotoxic T cells, and that BBD inhibitors synergize with paclitaxel and PD-L1 blockade in preclinical models. Two specific aims are proposed. In Aim 1, the effects of PARP inhibition alone and in combination with PD-1 blockade on the immune microenvironment and on tumor growth will be assessed in an animal model of BRCA- associated TNBC and in a clinical trial. Experiments will be conducted in mice bearing TNBCs derived from the K14Cre;BRCA1f/f;p53f/f genetically-engineered mouse model, and will translate to a phase 2 trial in the neoadjuvant setting using niraparib or combined niraparib/PD-1 therapy, in which changes in T cell infiltrate and pathologic complete response (pCR) rate will be defined. Preclinically, combined PARP inhibition and PD-1 blockade will also be investigated in BRCA wild-type syngeneic TNBC models, including EMT-6 and JC. In Aim 2, the effects of the BBD inhibitor JQ1 alone and in combination with PD-L1 blockade will be evaluated in the same syngeneic and genetically-engineered mouse models of TNBC used in Aim 1, as well as in a clinical trial. Changes in the composition and activation of components of the immune microenvironment will be assessed following JQ1 or JQ1 and PD-L1 treatment, with or without paclitaxel. Finally, a Phase 1 dose-escalation trial combining the JQ1 derivative RO6870810 and atezolizumab as a doublet, or with paclitaxel as a triplet, will be performed using concomitant and sequential schedules, in which tumor biopsies will be studied to document changes in the immune microenvironment and in copy number and expression of CD274, encoding PD-L1. The successful completion of this project will improve our understanding of the immune effects of these targeted therapies and may identify biomarkers to aid the selection of patients most likely to benefit.